1. Technical Field
This invention pertains to the field of digital communications systems, particularly to the field of circuit-switching systems such as digital cross-connects and add-drop multiplexers.
2. Descriptions of the Related Art
The following acronyms are used and referred to herein as follows:    ADM (SDH/SONET) Add-Drop Multiplexer    MSoB Multi-Source Bus    PS Protection Switch    SD Signal Degrade    SDH Synchronous Digital Hierarchy, ITU-T Recommendations G.707, G.783 and G.806    SF Signal Fail    SONET Synchronous Optical Network    STS-1 The base-level SONET signal. For XC purposes, equal to SDH VC-3.    STS-N A SONET signal carrying N (an integer) time-division-multiplexed byte interleaved STS-1 tributaries    TDM Time-Division Multiplexing    XC Cross-connect
Communications networks consist of terminal equipment, switches and communications paths (also called as connections, links or circuits) between the terminal equipment and the switches. Typically a switch has a number of input and output i.e. I/O ports, and a path may be routed through such a switch port. The term port is used herein in logical, rather than physical, sense, so that a number of (logical) ports may reside within a single physical interface line of a switch. The switches are able to connect data traffic from its input ports to its outputs ports. Certain type of switches, called circuit-switches, are able to connect paths from its input ports to its output ports. Connecting a path means connecting the signal carried by the path in its entirety from the input port to the output port occupied by the path. A circuit-switch that provides a such path level connectivity among its input and output ports is called a crossconnect (XC).
For network configuration purposes the input and output ports of such an XC system are typically identified by port numbers. E.g., if an XC node had one hundred I/O ports, these I/O ports of it could be identified as port #1 through port #100. The input-output port connection configuration, called an XC table, for such an 100-port XC node is regularly provisioned by the network operator so that per each output port #p (p=1,2, . . . ,100) is configured the port ID #q (q=1,2, . . . ,100) of the input port that is to be connected to it. Thus, in the case of the 100-port XC, if an input port #25 is configured to the XC table entry #50, corresponding to the XC output port #50, the path on the input port #25 would be connected to the output port #50 of the XC.
For traffic protection purposes the same traffic could be carried over more than a single path, typically over two diversely routed paths, from its source across a network to its destination. Thus, an XC could, per a single output port of it, receive two input paths that are provisioned to carry the same traffic stream, and the XC should connect, out of the two input paths, to that output port the traffic from that one of the two input paths that has better signal quality. For instance, in the case of the above 100-port XC, suppose that an additional path is provisioned across network to carry the same traffic stream to the XC as the path on the XC input port #25, and that protection path arrives to the XC at its input port #75. Then, if the input signal at port #25 fails, i.e. is under signal fail (SF) condition, or degrades in quality below a specified protection-switch threshold, i.e. is under signal degrade (SD), the signal from the protection path i.e. from input port #75, instead of input port #25, should the connected to the output #50, assuming the input path #75 is of better quality than input #25. The SF and SD conditions are defined for SDH signals in ITU-T Recommendations G.783 and G.841, and both cause a protection switch request called SFprot. This type of a process of changing the XC configuration with the purpose of continuing the traffic stream delivery is called protection switching (PS). A common standard based scheme for the herein discussed path-level protection is called Sub-Network Connection Protection (SNCP), which is defined in ITU-T Recommendations G.841 and G.783. For voice type of or other delay sensitive traffic forms, the protection switching process should be completed in less than 50 ms, as specified in telecommunications standards such as ITU-T Recommendation G.841, in order for the parties communicating over the network to not notice a significant disruption in their communication channel.
The protection-switch completion time objective of 50 ms (from the activation of SFprot) naturally applies for each individual output path of an XC. While performing protection-switching in a conventional fashion, i.e. by a shared microprocessor of the XC system, requires a finite amount of time per each output reconfigured, a protection-switch however could need to be done for any number, up to all, of the output paths of the XC node simultaneously, yet independently for each output path. Thus, if the XC system was able to complete the protection-switch for a single output path in e.g. 5 ms, it could meet the 50 ms PS completion time objective for at most for 10 output paths whose related input paths failed or degraded below the PS initiation criteria simultaneously. This problem currently seriously limits the maximum number of protected paths that can be supported by XC systems in telecommunications networks. It should be further noted that an XC should be able to perform the protection switching at individual path granularity (as opposed to collectively for all or none of its output paths), since it may serve both output paths that have as well as output paths that do not have protecting inputs, and also since some of the outputs may have their currently connected input paths of worse quality than their related protection inputs, while at the same time for some other outputs their currently connected inputs may be of better quality than their protection inputs.
Therefore, a cost-efficient mechanism that would enable an XC to perform a path-level PS within the required PS completion time, regardless of the number of outputs requiring simultaneous PS, is needed in order to improve the cost-efficient scalability and reliability of communications networks.